Natural gas is an abundant fossil fuel resource. Recent estimates places worldwide natural gas reserves at about 35.times.10.sup.14 standard cubic feet, corresponding to the energy equivalent of about 637 billion barrels of oil.
The composition of natural gas at the wellhead varies but the major hydrocarbon present is methane. For example the methane content of natural gas may vary within the range of from about 40 to 95 volume percent. Other constituents of natural gas may include ethane, propane, butanes, pentanes (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium and nitrogen.
Natural gas is classified as dry or wet depending upon the amount of condensable hydrocarbons contained in it. Condensable hydrocarbons generally comprise C.sub.3 + hydrocarbons although some ethane may be included. Gas conditioning is required to alter the composition of wellhead gas, processing facilities usually being located in or near the production fields. Conventional processing of wellhead natural gas yields processed natural gas containing at least a major amount of methane.
Processed natural gas, consisting essentially of methane, (typically 85-95 volume percent) may be directly used as clean burning gaseous fuel for industrial heat and power plants, for production of electricity, and to fire kilns in the cement and steel industries. It is also useful as a chemicals feedstock, but large-scale use for this purpose is pretty much limited to conversion to synthesis gas which in turn is used for the manufacture of methanol and ammonia. It is notable that for the foregoing uses, no significant refining is required except for those instances in which the wellhead-produced gas is sour, i.e., it contains excessive amounts of hydrogen sulfide. Natural gas, however, has essentially no value as a portable fuel at the present time. In liquid form, it has a density of 0.415 and a boiling point of minus 162.degree. C. Thus, it is not readily adaptable to transport as a liquid except for marine transport in very large tanks with a low surface to volume ratio, in which unique instance the cargo itself acts as refrigerant, and the volatilized methane serves as fuel to power the transport vessel. Large-scale use of natural gas often requires a sophisticated and extensive pipeline system.
A significant portion of the known natural gas reserves is associated with fields found in remote, difficultly accessible regions. For many of these remote fields, pipelining to bring the gas to potential users is not economically feasible.
Indirectly converting methane to methanol by steam-reforming to produce synthesis gas as a first step, followed by catalytic synthesis of methanol is a well-known process. Aside from the technical complexity and the high cost of this two-step, indirect synthesis, the methanol product has a very limited market and does not appear to offer a practical way to utilize natural gas from remote fields. The Mobil Oil Process, developed in the last decade provides an effective means for catalytically converting methanol to gasoline, e.g. as described in U.S. Pat. No. 3,894,107 to Butter et al. Although the market for gasoline is huge compared with the market for methanol, and although this process is currently used in New Zealand, it is complex and its viability appears to be limited to situations in which the cost for supplying an alternate source of gasoline is exceptionally high. There evidently remains a need for better ways to convert natural gas to higher valued and/or more readily transportable products.
A reaction which has been extensively studied is the direct partial oxidation of methane to methanol. This route, involving essentially the reaction of methane and gaseous oxygen according to the simple equation EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CH.sub.3 OH
could theoretically produce methanol with no by-product. However, further oxidation of some of the methanol product to CO and CO.sub.2 is usually observed. The homogeneous synthesis of methanol occurs most favorably under high pressure (10 to 200 atm.), moderate temperatures, (350.degree.-500.degree. C.), and at relatively low oxygen concentration to limit conversion. Oxidation of the methanol product to formaldehyde and to CO and CO.sub.2 are minimized under these conditions. The mechanism of methanol formation is believed to involve the methylperoxy radical (CH.sub.3 OO.) which abstracts hydrogen from methane. Up until now the per pass yields of methanol from direct partial oxidation have been limited. This limited yield has been explained as due to the low reactivity of the C-H bonds in methane vis-a-vis the higher reactivity of the primary oxygenated product, methanol, which results in increased formation of the deep oxidation products CO and CO.sub.2 when attempts are made to increase conversion.
Since it is known that methanol by itself can be catalytically converted to gasoline boiling range hydrocarbons with ZSM-5 type zeolites, the prospect of conducting this conversion during the direct partial oxidation of methane, i.e., redirecting the selectivity of the reaction from methanol to liquid hydrocarbons, is appealing. Previous investigators, however, have reported little success at significant redirection of selectivity.
Shepelev and Ione (React. Kinet. Catal. Lett., 1983, 23,323), found that mixtures of oxygen and methane in the presence of "ZSM" zeolites leads mainly to the formation of CO, CO.sub.2 and H.sub.2 O at 600.degree. C. and atmospheric pressure. It appeared possible, however, to detect traces of ethane, ethylene, hydrogen and benzene. Studies at 100 atmosphere pressure and 400.degree. C. indicated that a small amount of (unspecified) higher hydrocarbons formed. The methane and oxygen used were of 99.9% purity and reported free of higher hydrocarbons. However, U.S. Pat. 4,497,970 to D. Young discloses that such mixtures passed over zeolites under conditions similar to those used by Shepelev and Ione (ibid) formed only carbon oxides and water.